Apple supports up to 8 GB, but 16 GB is known to work. I think there's been one case of a software update breaking compatibility with "extended" memory so just be aware of that.

Edit: It was the 10.8.2 update, which didn't work on systems with more than 64 GB of RAM. Apple supports up to 64 GB in Mac Pro systems so everyone having this issue was running in an unsupported configuration. You'll probably be fine with 16 GB but I just feel that I should point it out.

Apple supports up to 8 GB, but 16 GB is known to work. I think there's been one case of a software update breaking compatibility with "extended" memory so just be aware of that.

Edit: It was the 10.8.2 update, which didn't work on systems with more than 64 GB of RAM. Apple supports up to 64 GB in Mac Pro systems so everyone having this issue was running in an unsupported configuration. You'll probably be fine with 16 GB but I just feel that I should point it out.

So your saying you could actually put 64GB of Ram in a Mac Pro?

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Thanks for all the help so far,

One more question,

I've just put the ram from the Mac Pro in the IMac, now I have.

2 X 2GB (Elpida)
2 X 2GB (Hynix)

Which brand is better quality performance wise? I would like to make sure that the better brand is in the two slots which get 1st priority.